Bob Harrington's reply to this post is correct. I'd like to add 2
additional bits of info
a) Here in the UK Cumulonimbus can be identified by shape alone.

The "tall towers" as Americans like to call them :-) can be seen from
many miles away- often too far to be sure if there is actual rain
falling from the base of the cloud. Cumulonimbus also often has a
distinctive "anvil" shape at the top, which helps identify them.
Because of their importance in terms of local flooding, wind shear,
damaging gusts, etc, we don't wait until we see rain (or lightning)
before classifying the cloud as Cumulonimbus (Cb).
We identify 2 types of CB - with anvil, or without anvil.

b) Your description of the 2 clouds together sounds very like a Cb and
"daughter cell".

Cold, dense, downdraughts from the Cb hit the ground and spread away
from the originating cloud. If the cold air collides with warmer air on
its way to being sucked in to the Cb, the result can be an uplifting of
the warm air. If it rises to the point at which the original cloud
formed, a second cloud (the daughter cell) might be formed.

As the inflow of air to the original cloud has been shut off, the
original often decays, and the daughter cell takes over as the main
cloud.

Hope this helps.

Colin